1. Field of the Invention
The present invention relates generally to electrical waveguides, and more particularly to waveguide filters for passing selected radio frequencies in electronic receivers and exciters.
2. Discussion of Related Art
Optimal electronic receivers must detect a broad range of radio frequencies (RF). The wide bandwidth exposes such receivers to an increased probability of receiving multiple signals simultaneously. In a radar application, these multiple signals could originate, for example, from electronic countermeasures of a hostile adversary, from other non-combative radiators, or from the radiator of the same radar system under certain receiving conditions. The simultaneous reception of such multiple signals can result in cross modulation, which degrades receiver performance.
Conventional techniques for decreasing the probability of cross modulation in receivers having wide bandwidths include the channelization of the receiver front end. Channelization involves dividing the receiver path into a series of channels using a series of filters configured in a switched filter bank with each filter tuned to a particular frequency band. Waveguide cavity, combine, and suspended stripline are a few examples of techniques that have been used to implement waveguide filters. Unfortunately, each of these prior structures lacks the advantages of small volume and light weight needed for many radar applications.
Waveguide cavity filters have achieved a high quality of electrical performance in passing selected radio frequencies and providing low insertion loss and high out of band isolation. These filters are constructed of a metallic shell having air-filled cavities formed by precision machining. Rectangular, square, and circular air-filled waveguides have been constructed to realize single mode or dual mode filter response. Cavity filters, however, are expensive to produce requiring special tuning to achieve desired performance, and are prohibitively large and heavy for certain applications.
One proposed technique has significantly reduced the size of dual mode waveguide filters by inserting a temperature stable ceramic material having a high dielectric constant and high quality factor into the cavities previously filled with air. This reduction resulted from the fact that the linear dimensions of a waveguide filter are inversely proportional to the square root of the effective relative dielectric constant within the waveguide. Despite the significant reduction in waveguide size, the proposed technique still required machining and tuning of the metallic shell and the placement of the dielectric within the metallic shell. These steps are labor-intensive and expensive under presently available manufacturing processes. Consequently, waveguide filters having cavities loaded with a dielectric resonator may not easily be integrated into a single package containing other electronic components, such as monolithic microwave integrated circuits (MMIC).
In light of the foregoing, there is a need for a miniature microwave filter that can be manufactured inexpensively, repeatedly, and in large quantities without the need for extensive tuning. In addition, there is a need for a waveguide filter that can be combined into a single integrated package with other electronic components.